1. Field of the Invention
The present invention relates to new binder compositions which are suitable for the production of solventless or low-solvent, one-component polyurethane stoving lacquers and to a process for the production of coatings on heat-resistant substrates using coating compositions containing the new binder composition.
2. Description of the Invention
The use of blocked isocyanates for the production of heat-curing polyurethane coatings is known (cf. for example Kunststoff-Handbuch, Vol. VII, Polyurethane, Carl Hanser Verlag Munchen, 1966, pages 11-13 and 21 et seq). A number of compositions suitable for the reversible blocking of isocyanate groups have been described. Among such compositions, particular significance is attributed to the class of C-H acidic compositions (such as malonic acid esters and acetoacetic acid esters) by virtue of their favorable chemical and physiological properties (cf. for example DE-OSS 2,342,603, 2,436,872, 2,550,156, 2,612,783, 2,612,784 and 2,612,785). Polyurethane stoving lacquers based on aliphatic polyisocyanates blocked with malonic acid esters or acetoacetic acid esters and organic polyhydroxyl compositions are described in DE-OS 2,550,156, DE-AS 2,623,081 and DE-AS 2,639,491.
The polyurethane stoving lacquers according to the cited publications are unsuitable for the production of high-impact coatings. However, there is an increasing demand for coating compositions that are suitable for the production of high-impact, chip resistant coatings for protecting the surfaces of industrial goods such as machine parts, vehicle bodies or transport containers. An improvement in this regard is disclosed in EP-A-0, 053,766 which is directed to systems suitable for the production of chip resistant fillers.
All previously known one-component systems based on blocked polyisocyanates and organic polyhydroxyl compositions are attended by the disadvantage that either they must contain at least 40% solvent for application or they are based on aromatic polyisocyanates which turn yellow when the coatings are exposed to heat.
In addition to the above-mentioned one-component polyurethane systems, compositions of blocked isocyanates with organic diamines are disclosed in DE-AS 2,131,299. Although these systems guarantee highly elastic surface protection, they are attended by the disadvantage that they only have a limited pot life of at most a few days, i.e., they are only "quasi one-component resins." In addition, low molecular weight amines can be released during the stoving process which can give rise to odor emissions and, in addition, can be toxicologically harmful.
Accordingly, it is an object of the present invention to provide new binder compositions which
1) are suitable for the production of fillers, intermediate primers or thick chip resistant coatings for automobiles, PA0 2) are not be attended by the disadvantages of known compositions, PA0 3) are suitable for the production of high-impact, highly elastomeric coatings of the type required for the protection of vehicle bodies, machine parts, transport containers or other industrial goods, PA0 4) are toxicologically safe, PA0 5) are suitable for the production of coating compositions which could readily be processed despite solids contents of more than 60% by weight, preferably more than 70% by weight, and PA0 6) are suitable for the production of coatings which are resistant to yellowing on exposure to heat because binders for fillers and intermediate primers must not turn yellow, particularly when coated with a clear lacquer. PA0 a) 20 to 70% by weight of oligomerization products of 1,6-diisocyanatohexane, the oligomerization products having isocyanate groups at least partially blocked with blocking agents for isocyanate groups and containing at least 50% by weight, based on the weight of the oligomerization products, of blocked uretdione diisocyanates corresponding to formula (1) ##STR3## wherein R represents the residue formed by removal of the active hydrogen atom from a monofunctional blocking agent, and PA0 b) 30 to 80% by weight of a polyol component containing a mixture of PA0 b1) 100 parts by weight of an alkoxylation product of bisphenol A corresponding to formula (II) ##STR4## wherein R' is hydrogen or a methyl group and PA0 n on average is a whole or fractional number from 1 to 2, PA0 b2) 20 to 200 parts by weight castor oil and PA0 b3) 0 to 40 parts by weight other organic polyhydroxyl compounds.
These objects may be achieved in accordance with the binders of the present invention as described in detail hereinafter.